Optical fibers are gaining widespread use in many applications and particularly for use in communications systems. Basically, an optical fiber comprises an inner core fabricated from a dielectric material having a certain index of refraction and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. In most practical applications, the refractive indices of the core and cladding differ from each other by only a few percent. In any case, as long as the refractive index of the core exceeds that of the cladding, a light beam launched into the fiber and propagated along the core exhibits total internal reflection and is guided along the length of the core to transmit a signal. For typical transmission applications, a fiber optic core comprises 3% GeO.sub.2 /SiO.sub.2 which at 25.degree. C. exhibits a refractive index of about 1.451 at the wavelength 1300 nm and a refractive index of about 1.448 at the wavelength 1550 nm. The refractive index is temperature dependent.
Similar to electrical transmission paths, it is important that optical transmission paths be properly connected to other paths or a termination port. A widely used connector for this purpose comprises the ST.RTM. connector, ST.RTM. being a registered Trademark of AT&T Corp., now Lucent Technologies, Inc. The design of this connector is described in U.S. Pat. No. 4,934,785, issued Jun. 19, 1990, and in U.S. Pat. No. 5,619,610, issued Apr. 8, 1997, both of which are hereby incorporated by reference. The latter of these patents ('910), entitled "Optical Terminator," issued to King and Lambert, two of the inventors herein, was assigned to Lucent Technologies, Inc., the assignee herein, and will herein be referred to as the King patent.
A difficulty in connecting and terminating optical transmission paths involves back reflection, that is, a signal may be reflected from a point of discontinuity back toward its source. A point of discontinuity resulting in back reflection may occur for a number of reasons, such as a change in the refractive indices of interfacing materials, misalignment of the cores of optical fibers being connected, or perturbations along the transmission path. A back-reflected signal also may be further reflected at the source to be retransmitted along the transmission path. Back reflections present a serious problem in optical fiber systems as they introduce undesirable noise components into the signal, degrade performance of the system capacity, and can corrupt the transmission source, which typically is a laser.
Reflectance can be calculated as a function of the refractive index differential of interfacing materials, pursuant to the equation: EQU Reflectance (dB)=10 log [(n.sub.o -n.sub.i).sup.2 /(n.sub.o +n.sub.i).sup.2 ]
Thus, for a glass-to-air interface (n.sub.o .apprxeq.1.0 for air, n.sub.i .apprxeq.1.46 for glass), the reflectance is about 14.6 dB. For two materials with only slightly different indices of refraction (e.g., n.sub.i .apprxeq.1.46 vs. 1.47), the reflectance is calculated to be -49.3 dB. Optimally for high performance systems, reflectance generated by an optical connection should be less than -50 dB.
It is important to control back reflections associated with optical fiber connectors and terminators, and many approaches have been taken to this end. One approach involves interfacing the end portion of the optical fiber with a terminator or attenuator portion that is index matched to the core of the optical fiber, thereby avoiding a glass-to-air interface. U.S. Pat. No. 4,998,795 issued to Bowen et al. on Mar. 12, 1991, entitled "Reflection-Less Terminator," which is incorporated herein by reference, describes use of a carbon-black filled epoxy material in fabricating a termination portion which also is designed to absorb the radiation and dissipate the signal. It is difficult to precisely match, however, the index of refraction of the core with the material used in making the termination, and use of anti-reflective surface coatings may be necessary. But anti-reflective surface coatings present additional problems in that when exposed to aging environments, the coatings may crack, delaminate, and flake, thereby undermining the effectiveness of the coating and the reliability of the device.
A low reflection attenuation device for use in an optical fiber connector is described in U.S. Pat. No. 5,082,345 issued to Cammons et al. on Jan. 21, 1992, entitled "Optical Fiber Connecting Device Including Attenuator" (the "Cammons patent") assigned to AT&T Bell Laboratories, a predecessor of Lucent Technologies, Inc. (the assignee herein), which is hereby incorporated by reference. The Cammons patent describes use of polymethylmethacrylate (PMMA) to fabricate a disc-shaped attenuator portion (FIG. 2, 70). However, PMMA has an index of refraction of 1.49, whereas a fiber optic core is typically fabricated with 3% GeO.sub.2 -doped silica having a refractive index of about 1.451 at 25.degree. C. and 1300 nm. This refractive-index differential correlates to an attenuator portion producing -40 dB reflectance which is suitable for many applications but less than optimal for high performance optical fiber systems.
A low-reflection terminator fabricated from a polymeric material having an index of refraction of about 1.45.+-.0.01 and enabling reflectance of less than -50 dB is described in the previously referenced King patent. Use of polymeric materials to fabricate the attenuators and terminators is advantageous because polymers are inexpensive (as compared with ceramics), and may be readily molded into desired configurations with use of injection molding. The attenuation level is determined by the thickness of the element (which typically is from 0.010 to 0.060 inches), and by the incorporation of absorbing particles (carbon black) or scattering particles; a small percentage of carbon black or scattering particles may be added to the polymer to contribute an absorptive component and thereby increase the attenuation level. However, there are a limited number of thermoplastic polymers that may be used in optical attenuators and terminators meeting all the desired criteria of having a refractive index of about 1.45.+-.0.01 enabling reflection of -50 dB, allowing for injection molding, and also having a low creep modulus.
Low creep modulus is important in fabricating connector components of optical attenuators to ensure reliable long-term performance. Thermoplastic polymers will irreversibly creep when subjected to compressive forces and/or high temperatures such as those encountered in optical attenuators. In application, an optical attenuator will be pressed against an optical fiber for long periods of time, ultimately resulting in indentations on the surface of the connector element. These surface deformations will impact upon the performance of the device when different connections are made. The extent of the creep and, therefore, the extent or size of the surface deformations will depend on the applied force, the use temperature, and the glass transition and heat distortion temperature of the polymer used to fabricate the connector elements. The heat distortion temperature reflects the temperature at which significant distortions occur and is measured at 264 psi according to standards known in the field as ASTM D648. The higher the heat distortion temperature of the polymer, the more resistant the material will be to deformation. Advantageously, the heat distortion temperature should be greater than about 80.degree. C. and even more preferably above 100.degree. C.
The previously referenced King patent describes an optical terminator fabricated with use of acrylics, e.g., PMMA which are advantageous in terms of their optical properties but have less than optimal heat distortion temperatures. For example, Acrylite,.TM. which is a trade name for a PMMA product available from Cyro Industries, has a heat distortion temperature of 90.degree. C. The heat distortion temperature of the acrylics can be raised upon blending with other compounds such as polyimide, polyvinylidene fluoride, and polymethylpentene polymers. A suitable acrylate-polyimide blended material was formerly available from ELF Atochem under the trademark Kamax,.TM. and a copolymer of propylene and 4-methyl-pentene-1 is commercially available under the trade name TPX.TM. from Mitsui Plastics. However, blending the materials with compounds to increase the heat distortion properties may unacceptably alter the refractive indices of the materials, and vice-versa. Synthesizing a thermoplastic polymer or co-polymer for use in an optical attenuator or terminator application which optimizes both the optical and mechanical properties presents a difficult challenge.
Accordingly, there remains a need for an optical attenuator or terminator element fabricated with alternative materials and, in particular, with materials meeting the desired criteria of having a refractive index of about 1.45.+-.0.06, enabling reflection of about less than -50 dB, allowing for easy manufacturing, and also resistant to permanent deformation. This invention addresses these needs. Further advantages may appear more fully upon considering the description below.